Astrocytic Ca2Â + waves mediate activation of extrasynaptic NMDA receptors in hippocampal neurons to aggravate brain damage during ischemia

Excitotoxicity plays a central role in the neuronal damage during ischemic stroke. Although growing evidence suggests that activation of extrasynaptic NMDA receptors initiates neuronal death, no direct evidence demonstrated their activation during ischemia. Using rat hippocampal slices, we detected oxygen-glucose deprivation (OGD) induced slow inward currents (SICs) mediated by extrasynaptic NMDA receptors in CA1 pyramidal neurons. Moreover, Ca2 + chelator BAPTA dialysis into astrocytic network decreased the frequency of OGD induced SICs, indicating that the activation of extrasynaptic NMDA receptors depended on astrocytic Ca2 + activity. To further demonstrate the importance of astrocytic Ca2 + activity, we tested hippocampal slices from inositol triphosphate receptor type 2 (IP3R2) knock-out mice which abolished the astrocytic Ca2 + activity. As expected, the frequency of OGD induced SICs was reduced. Using two-photon Ca2 + imaging, we characterized the astrocytic Ca2 + dynamics. By controlling Ca2 + level in the individual astrocytes using targeted photolysis, we found that OGD facilitated the propagation of intercellular Ca2 + waves, which were inhibited by gap junction blocker carbenoxolone (CBX). CBX also inhibited the Ca2 + activity of the astrocytic network and decreased the SIC frequency during OGD. Functionally, the infarct volumes from brain ischemia were reduced in IP3R2 knock-out mice and in rat intracerebrally delivered with CBX. Our results demonstrate that enhanced Ca2 + activity of the astrocytic network plays a key role on the activation of extrasynaptic NMDA receptors in hippocampal neurons, which enhances brain damage during ischemia.